The present invention relates to a static convergence device for in-line electron beams emitted from three guns located at a bottom in a color picture tube.
A static convergence assembly (STC) for use with an in-line color cathode ray tube (Color-CRT) has an effect to make electron beams emitted from guns located at a bottom of a tube, strike coincident regions of a viewing screen.
A STC comprises a pair of juxtaposed four-pole magnet rings and a pair of justaposed six-pole magnet rings which are rotatably mounted about axially spaced regions of the tube neck. The described ring magnet structures may advantageously be used in combination with a pair of juxtaposed two-pole magnet rings called purity rings, each magnetized across a diameter of a ring for effecting movement of all three beams in the same direction, for accomlishing both purity and convergence adjustments. These prior art devices are described in U.S. Pat. No. 3,725,831, British Patent Specification No. 1,429,292 and Japan Patent Publication No. 55-30659.
Previously, four-pole magnet rings and six-pole magnet rings in a STC were composed of a mixture of Ba-ferrite magnetic material and resin material which was called a Ba-ferrite type of plastic magnet.
However, the above-mentioned STC had drawbacks in that the magnet rings caused an undesirable discrepancy of beams on a viewing screen after a temperature change in a STC even after focusing three beams to a point on the viewing screen by adjustments to the STC. The undesirable discrepancy of beams after a temperature change in a STC occurred by reason of the magnetization of the magnet rings comprised of Ba-ferrite type plastic magnet material in the STC which were reduced or enhanced too much by a temperature change of the STC, because the Ba-ferrite type plastic magnet material had rather big temperature coefficient of magnetization as -0.2%/.degree.C.
The temperature coefficient (.alpha.) of magnetization as used in the specification and drawings of the present invention is defined by a formula: ##EQU1## where .PHI.b is the magnetic flux (Maxwell) of the permanent magnet at a temperature of Tb.degree.C. and .PHI.a is the magnetic flux (Maxwell) of the permanent magnet at a temperature of Ta.degree.C.
Recently such an undesirable beam discrepancy on a viewing screen which is caused by a rather big temperature coefficient of magnetization of magnet rings in a STC becomes more and more serious in current state of the art high-performance color picture tubes.